Infusion extraction (also called "leaching", "washing extraction", "diffusional extraction", and "solvent extraction") is the preferential solution of one or more constituents of a solid mixture by contact with a fluid solvent. As used in this application, it may also include the washing or carrying away of constituents from the surface of insoluble material and, in addition to physical solution, per se, it may include physical dissolution or dissolution by chemical reaction and, additionally, includes the release of volatiles and aromatics from both soluble and insoluble matter by physical and or chemical action of the solvent. While the term "solvent" is used throughout this application it is to be understood that, depending on the process and the infusible material being extracted, the term may apply not only to water but also to steam, ethyl and isopropyl alcohol, petroleum napthas, hexane, trichlorethylene, acetone, perchlorothylene, chlorinated hydrocarbons, and other solvents used in the various extraction processes and, in addition, the term "solvent" as used in this application is not limited to solely the dissolving properties of a fluid but includes also any other fluid properties, physical or chemical, useful in infusion extraction such as, for example the physical washing of constituents by the fluid as well as osmotic mass transfer across membranes.
Infusion extraction processes usually involve the general steps of: reducing the size of particles of the infusible material, infusion extraction of the infusible material with a solvent, and separating the resulting extract and extraction residue.
Successful infusion extraction processes involve appropriate preparation of the solid infusible material prior to the infusion. Since in some cases small particles of the soluble material are surrounded by a matrix of insoluble-matter, the solvent must diffuse (as by osmosis) into the mass of material and the resulting solution or extract must diffuse out. Crushing and grinding of such solids will greatly accelerate the action. Vegetable and animal bodies are cellular in structure, and the natural products to be extracted from these materials are usually found inside the cells. Grinding, crushing, or rolling to various sizes are sometimes useful in rupturing the cell walls and thus exposing the material to be dissolved or washed in the extraction process. Different "grinds" of coffee, have been found preferable for different brewing processes. Grinding, however, hastens volatility. Ground coffee, for example begins to lose freshness when exposed to the air because of oxidation of the chemical ingredients that provide the flavor and aroma. The vacuum packing of coffee after grinding has, therefore, been practiced for many years. The flavor and fragrance industry utilizes the extracts and distillates of many natural products (flowers, leaves, stems, roots, fruits, etc.). These serve as key ingredients for the preparation of compounded flavors and fragrances. A problem facing aromatic extract producers is that many of the more important aromatic components of natural products are quite volatile and exist in extremely small concentrations i.e., parts per million and even parts per billion (Teraniski, et al, "Flavor Research, Principles and Techniques", Marcell Dekker, Inc., 1971, p.39). An extraction process wherein the infusible material remains hermetically sealed prior to and during extraction would, therefore, be advantageous.
Some ground material, e.g., coffee, swells when it absorbs liquid slowing the percolation of gravity-fed devices and if pressure is used to force liquid through the ground matter it is thereby compacted, thus even further reducing flow rates. Preferred routes, "tunnels" or "channels", may even be formed in the batch when water is allowed or forced to flow through the batch without agitation. In preferred routes or channels, a disproportionate amount of the liquid passes through the channels and the soluble constituents are leached from the particles adjacent to the channel depleting them early, and liquid later passing through the channels thus does not extract as much, while still other particles not adjacent to channels remain virtually unleached (i.e., there is no uniformity of contact between solvent and infusible material). That is, devices which pass solvent through the batch of infusible material negate the well known benefits of "agitation." Therefore, mixing liquid solvent and infusible solid material in a receptacle so that infusible material is suspended in this solvent, and then withdrawing the liquid extract may be preferable to forcing liquid through the infusible material. It would thus be advantageous to allow solvent to be moved into infusible material, agitating it and suspending it in the solvent, and thereby assuring more uniform contact with, and infusion extraction by, the solvent.
It is well known that relatively higher temperatures sometimes accelerate the infusion extraction process and generally the higher the temperature the faster and more complete the process and the higher the concentration of the extract. The increased rate is due, among other things, to lower viscosity when a liquid solvent is used. However, it is possible to get temperatures too high for the particular infusion material, sugar beets for example, yield undesirable solutes or cause chemical deterioration of the product if processed at too high a temperature. In other words, for any given product there is an optimum temperature or temperature range. For example, the non-profit Coffee Brewing Institute, Inc. of the Pan American Coffee Bureau, has recommended 195.degree. F..+-.5.degree. F. as the optimum water temperature for brewing coffee. It is desirable therefore, to permit close temperature control of the solvent-infusible material mixture.
Another important parameter of the infusion extraction process is the "steep" time or length of time the solvent is maintained in contact with the infusible material. In the case of coffee and tea, for example, too long a contact time results in a beverage which is too strong and bitter, whereas too short a contact time yields a weak drink. The time of infusion is critical for other products as well, and the time of percolation due to gravity, since it is dependent on a number of factors (particle size, geometric configuration of container, etc.) may be too long or too short for the optimum infusion extraction in a particular process. Spices, for example, are derived from various parts of plants; leaf, flower, fruit, and barks and while some are used in their natural state (after drying and grinding), there is quite a market for spice extracts (generally oleoresins). The oleoresins are usually separated by use of a solvent in infusion extractions and usually by gravity percolation of solvent through the ground solid. Various solvents may be used depending on the particular spice. Time of contact with solvent is a sensitive parameter determining not only the concentration of the extract but its composition as well, the solubility of some constituents depending on time of contact with the solvent as well as its temperature. The rate at which infusion extraction takes place depends on a number of variables of which incomplete knowledge is available at this time. The washing of a solution from the surface of impervious solid particles may be expected to be very rapid, requiring only the mixing of infusible material and solvent, whereas, leaching of a solute from the internal parts of a solid will be relatively slow. The rate of dialysis or osmosis, dissolving of solute from matrix of insoluble matter, and rate of chemical reaction etc. will all differ for different processes. It is well known, for example, that the properties of soybean oil differ depending on the relatively longer or shorter leaching times utilized. (Treybal, "Mass-Transfer Operations", McGraw-Hill, 1955, p628). The time required for percolation of a liquid through a ground solid depends, of course, on such things as the particular solid and liquid and the particle size and size of the batch, but for a given batch and solvent, the steep or contact time during percolation is of relatively fixed duration. It is desirable therefore, to facilitate close control over the time of contact of solvent with infusible material.
The ability to vary the pressure of the solvent/infusible material mixture may be advantageous for obtaining beneficial results, for example, to assist in breaking cell walls of infusion material. Also, pressurizing the solvent/batch mixture followed by a vacuum removal of the extract may facilitate collection of volatiles not otherwise released. It is advantageous to provide the capability to apply and control a desired pressure to aid the infusion extraction process.
It is apparent, therefore, that a number of variables; type of solvent, steep time, temperature, pressure and degree of agitation; have a significant influence on not only the speed and efficiency of the extraction process but also on the constitution of the extract itself. The ability to vary each, or any combination of these variables would be advantageous in a number of ways, for example, experimentation on a small scale to determine the optimum combination of parameters for extraction of a given infusible material before scaling up to produce it commercially.